Conventional accelerometers contain moving parts and detect specific forces to ascertain acceleration. For example, a pendulous integrating gyroscopic accelerometer (PIGA) includes a spinning gyroscope and a pendulous mass mounted on a bearing. A spring-mass accelerometer employs a proof mass suspended within a frame by one or more springs and measures displacement of the proof mass, relative to the frame, as a result of acceleration. Moving parts make such conventional accelerometers difficult to manufacture and subject to mechanical wear.
U.S. Pat. No. 7,324,205 to Robert J. Howard (“Howard”) discloses a device, said to be an optical accelerometer, gravitometer and gradiometer. However, Howard's device requires “slow light,” i.e., light that travels at significantly less than c, the speed of light in a vacuum. Slow light may result from light passing through a medium that has been cooled to near absolute zero to produce a Bose-Einstein condensate (BEC), which causes electromagnetically-induced transparency (EIT) of an ordinarily opaque material. Howard's device requires cumbersome apparatus to produce slow light. Furthermore, Howard's device is subject to Sagnac-effect corruption.
In his 1868 paper “Determination de la vitesse avec laquelle est entrainée une onde lumineuse traversant un milieu en movement” (“Determination of the Speed with which a Light Wave is Entrained Crossing a Moving Medium”), Martin (also Martinus) Hoek (“Hoek”) disclosed an unbalanced interferometer.
U.S. Pat. No. 6,813,006 to Ruyong Wang, et al. (“Wang”) improves on the Hoek device and discloses an optical device said to measure speed of an object by detecting interference between two electromagnetic beams that pass through two media having different dispersive dragging effects on the beams. However, Wang's description of his device as a “velocimeter” is believed to be incorrect, as his device is not sensitive to velocity. Instead, Wang's device is sensitive to acceleration, i.e., change in velocity between successive inertial velocity states.